The present invention relates to a method of and an apparatus for the handling and precise positioning of radioactive sources used in radiation oncology and intravascular radiotherapy, particularly to a device known as an afterloader, which advances a wire or cable having a radioactive source at the tip along a catheter or other closed pathway to a position within the body of a patient for a predetermined period of time and which thereafter withdraws the wire and radioactive source from the patient.
It is known in the medical field to use afterloader devices in the treatment of cancerous tumors using radioactive sources having intensity greater than that which can safely be handled. Typically one or more catheters, needles, or other closed pathways (hereafter "catheters") to the treatment site are positioned in the patient. The catheters are then attached to the afterloader which advances the radioactive source at the end of the wire, sometimes called a sourcewire, along the catheters according to a predetermined sequence calculated to deliver a therapeutic dose of radiation to the tumor. Typical of the prior art apparatus are those disclosed in U.S. Pat. Nos. 4,631,415; 4,881,937; and 5,030,194. Many of these prior art devices advance the sourcewire by means of a friction drive belt trained about a wheel with the wire sandwiched between the belt and wheel.
Less well known but rapidly gaining acceptance is the use of radiation to prevent or inhibit restenosis following percutaneous transluminal coronary angioplasty (PTCA) or other arterial lumen opening procedure. PTCA, also known as balloon angioplasty, is the predominant treatment for coronary vessel stenosis. Approximately 300,000 procedures were performed in the United States (U.S.) in 1990 and an estimated 400,000 in 1992. The U.S. market constitutes roughly half of the total market for this procedure. The increasing popularity of the PTCA procedure is attributable to its relatively high success rate, and its minimal invasiveness compared with coronary by-pass surgery. Patients treated by PTCA, however, suffer from a high incidence of restenosis, with about 35% of all patients requiring repeat PTCA procedures or by-pass surgery, with attendant high cost and added patient risk. More recent attempts to prevent restenosis by use of drugs, mechanical devices, and other experimental procedures have had limited success.
Restenosis occurs as a result of injury to the arterial wall during the lumen opening angioplasty procedure. In some patients, the injury initiates a repair response that is characterized by hyperplastic growth of the vascular smooth muscle cells in the region traumatized by the angioplasty. The hyperplasia of smooth muscle cells narrows the lumen that was opened by the angioplasty, thereby necessitating a repeat PTCA or other procedure to alleviate the restenosis.
Preliminary studies indicate that intravascular radiotherapy (IRT) has promise in the prevention or long-term control of restenosis following angioplasty. It is also believed that IRT may be used to prevent stenosis following cardiovascular graft procedures or other trauma to the vessel wall. A proposed IRT method disclosed in copending application Ser. No. 08/644,101 assigned to the assignee of this invention is first to advance a flexible catheter (radioguide catheter) through the cardiovascular system of the patient until the distal tip is at or near the region of the vessel that has been subjected to the angioplasty procedure. Subsequently, a sourcewire is advanced, preferably by an afterloader, along the radioguide catheter until the radiation source is disposed at the affected region. The radiation source is held at the affected region for a predetermined treatment period calculated to deliver an effective dose of radiation, then is withdrawn.
It will be appreciated from the foregoing that highly accurate positioning of the source within the patient is essential to maximize the effectiveness of the treatment while minimizing the damage to adjacent healthy tissue. It will also be appreciated that the source must be advanced to the treatment site as quickly as possible to minimize injury to healthy tissue along the catheter leading from outside the body of the patient to the treatment site.
To minimize trauma to sensitive tissue, the catheters and sourcewires that are used in sensitive areas are chosen to be as small as practicable, typically on the order of 0.5 millimeters. Use of these small diameter sourcewires presents special problems for the afterloader, for the small diameter wire does not have sufficient column strength to be driven into the catheter unless the afterloader design incorporates special precautions to prevent wire buckling. These problems associated with the potential buckling of the sourcewire are compounded by the need for rapid advancement of the sourcewire to avoid damaging healthy tissue.
With respect to the IRT application, in order to reach the site where the PTCA has been performed, the IRT sourcewire must often follow a tortuous pathway through the narrow twisted openings of the coronary arteries. In order to avoid blocking blood flow in these narrow openings, use of the smallest possible radioguide catheter and sourcewire is often required. If, however, the tiny radioguide catheter becomes kinked or otherwise obstructed as it is implanted, unless the obstruction is detected, the afterloader may drive the sourcewire through the wall of the catheter and even through the wall of the patient's blood vessel, with dire consequences. This problem is solved by the use of an active force feedback to enable the afterloader to drive a sourcewire through a catheter or other pathway at the highest possible speed without risk of puncturing a catheter or buckling the sourcewire as disclosed in copending U.S. patent application Ser. No. 08/436,075, the disclosure of which is incorporated herein by reference.
Another problem with prior art afterloaders is that the sourcewires in the afterloaders typically must be replaced by highly trained technicians. Such technicians must be periodically called upon to load the replacement sourcewire into the afterloader and verify the proper functioning of the afterloader system. In the case of short half-life sources, frequent replacement of sourcewires by skilled technicians obviously represents a significant cost in the maintenance of an afterloader. One solution to this problem is disclosed in the aforesaid copending U.S. application Ser. No. 08/436,075. That solution involves the use of a replaceable modular "cassette" in which most of the components of the wire storage and wire drive systems are mounted in a cartridge that can be readily detached from the afterloader housing and replaced with a new cartridge. The sourcewire is loaded into the cartridge at the manufacturer and the proper operation of all the drive mechanisms, monitoring devices and other key components is verified by the manufacturer before the cartridge is delivered to the user. In this way, the cartridge can be installed in the afterloader at the user's facility by relatively unskilled persons with confidence that the afterloader system will function correctly.
It is important that such a replacement cartridge be relatively compact and lightweight for ease of handling, and have a rugged, highly reliable design to permit afterloader installation by unskilled persons and especially to minimize premature failures before sourcewire replacement is required.